Global sea-to-air flux climatology for bromoform, dibromomethane and methyl iodide

Abstract

Volatile halogenated organic compounds containing bromine and iodine, which are naturally produced in the ocean, are involved in ozone depletion in both the troposphere and stratosphere. Three prominent compounds transporting large amounts of marine halogens into the atmosphere are bromoform (CHBr₃), dibromomethane (CH₂Br₂) and methyl iodide (CH₃I). The input of marine halogens to the stratosphere has been estimated from observations and modelling studies using low-resolution oceanic emission scenarios derived from top-down approaches. In order to improve emission inventory estimates, we calculate data-based high resolution global sea-to-air flux estimates of these compounds from surface observations within the HalOcAt (Halocarbons in the Ocean and Atmosphere) database (<a href="https://halocat.geomar.de/"target="_blank">https://halocat.geomar.de/</a>). Global maps of marine and atmospheric surface concentrations are derived from the data which are divided into coastal, shelf and open ocean regions. Considering physical and biogeochemical characteristics of ocean and atmosphere, the open ocean water and atmosphere data are classified into 21 regions. The available data are interpolated onto a 1°&times;1° grid while missing grid values are interpolated with latitudinal and longitudinal dependent regression techniques reflecting the compounds' distributions. With the generated surface concentration climatologies for the ocean and atmosphere, global sea-to-air concentration gradients and sea-to-air fluxes are calculated. Based on these calculations we estimate a total global flux of 1.5/2.5 Gmol Br yr⁻¹ for CHBr₃, 0.78/0.98 Gmol Br yr⁻¹ for CH₂Br₂ and 1.24/1.45 Gmol Br yr⁻¹ for CH₃I (robust fit/ordinary least squares regression techniques). Contrary to recent studies, negative fluxes occur in each sea-to-air flux climatology, mainly in the Arctic and Antarctic regions. "Hot spots" for global polybromomethane emissions are located in the equatorial region, whereas methyl iodide emissions are enhanced in the subtropical gyre regions. Inter-annual and seasonal variation is contained within our flux calculations for all three compounds. Compared to earlier studies, our global fluxes are at the lower end of estimates, especially for bromoform. An under-representation of coastal emissions and of extreme events in our estimate might explain the mismatch between our bottom-up emission estimate and top-down approaches

Estimates of tropical bromoform emissions using an inversion method

“…For the tropics defined as 20 • S-20 • N, these emissions include ∼ 240 Gg CHBr 3 yr −1 , ∼ 310 Gg CHBr 3 yr −1 (Warwick et al, 2006, updated by Pyle et al 2011 and ∼ 300 Gg CHBr 3 yr −1 (Ordóñez et al, 2012). Our estimate, however, is somewhat higher than the recent "bottom up" estimate of Ziska et al (2013), who used two different statistical methods to arrive at a tropical flux of ∼ 70-90 Gg CHBr 3 yr −1 . We also emphasise that the spatial distribution of CHBr 3 emissions in our "fine" grid cells is not similar to the distribution of either chlorophyll a or ocean depth.…”

“…Hossaini et al (2013) have recently compared the impact of including the four other estimates discussed above in the same global model, and found that no single inventory allows a satisfactory match with observations in all locations. For the tropics, they found that the relatively low emissions of Ziska et al (2013) provided the best match with the few available datasets. The paucity of tropical data, added to the fact that the different studies used different subsets of those data to construct their inventories, is likely to be one cause of the differing tropical emission values.…”

“…Warwick et al, 2006;Hossaini et al, 2010) have converged on a value of ∼ 60 Gg yr −1 (e.g. Liang et al, 2010;Ordóñez et al, 2012;Ziska et al, 2013). One part of the world where CHBr 3 emissions are particularly poorly constrained is the Maritime Continent, where long-term observations have only recently become available (e.g.…”

“…In the "bottom-up" method local flux calculations are assumed to be representative of, and are extrapolated over, particular oceanic domains (e.g. Quack and Wallace, 2003;Carpenter and Liss, 2000;Butler et al, 2007;Ziska et al, 2013). In the "top-down" method an idealised pattern of emissions is typically assumed based on available observational evidence and the emission magnitude is then varied to match observations made in the "background" atmosphere (e.g.…”

How sensitive is the recovery of stratospheric ozone to changes in concentrations of very short-lived bromocarbons?

“…Although their tropospheric lifetime is relatively short (a few weeks for CHBr 3 and several months for CH 2 Br 2 ), they can be lifted effectively through deep convective systems and transported into the upper troposphere and/or lower stratosphere (UTLS) to make a significant contribution to the total ambient bromine (Sturges et al, 2000;Yang et al, 2005;Salawitch, 2006). Being short lived, there are large uncertainties in using atmospheric concentration measurements to estimate their global fluxes and their net contribution to the stratospheric bromine (Warwick et al, 2006;Liang et al, 2010;Pyle et al, 2011;Ordóñez et al, 2012;Ziska et al, 2013;Hossaini et al, 2013). A large range of contributions to stratospheric inorganic bromine of 110 ppt can be found in the literature (Dorf et al, 2008;Salawich et al, 2010;Schofield et al, 2011;Aschmann et al, 2011;Tegtmeier et al, 2012;Stachnik et al, 2013;Hossaini et al, 2012a).…”